The speeds of metal-oxide-semiconductor (MOS) transistors are closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Compound semiconductor materials of group III and group V elements (commonly known as III-V compound semiconductor) are good candidates for forming NMOS transistors for their high electron mobility. Therefore, III-V compound semiconductors were used to form NMOS transistors. To reduce the manufacturing cost, methods for forming PMOS transistors using III-V compound semiconductors are explored. FIG. 1 illustrates a conventional transistor incorporating III-V compound semiconductors. In the formation process, a plurality of layers is blanket formed on a silicon substrate, wherein the plurality of layers includes a buffer layer formed of GaAs, a graded buffer formed of InxAl1-xAs (with x between, but not equal to, 0 and 1), a bottom barrier formed of In0.52Al0.48As, a channel formed of In0.7Ga0.3As, a top barrier formed of In0.52Al0.48As, an etch stop layer formed of InP, and a contact layer formed of In0.53Ga0.47As. A first etch is performed to etch through the contact layer and stopping at the etch stop layer to form a first recess. A second etch is then performed to etch through the etch stop layer and etch into a portion of the top barrier to form a second recess. A gate, which is formed of metal, is then formed in the second recess. The resulting transistor has the advantageous features resulting from the quantum well being formed of the bottom barrier, the channel, and the top barrier.
The above-described transistor, however, suffers drawbacks. It is difficult to dope impurities into III-V compound semiconductors to a high impurity concentration. For example, GaAs may be implanted or in-situ doped with silicon as an impurity, while the maximum doping concentration of silicon is only between about 1017/cm3 and about 1018/cm3. In addition, the effective density of states in the conduction band of GaAs is only about 4.7×1017/cm3. The low density of states in the conduction band results in a high source/drain resistance, which in turn prevents the further improvement in the drive current of the resulting transistor. A method and structure for overcoming the above-described shortcomings in the prior art are thus needed.